Acceleration sensors are produced using micromechanical differential capacitors, among other things. In this context, a capacitive signal change is caused by the deflection x of the common electrode (designated as seismic mass in the following). Deflection x may be created by accelerations acting from outside, or by electrostatic field forces applied within the system. For partial capacitances C1 and C2, the capacitive change amounts to:C1=ε*A/(d+x), C2=ε*A/(d−x).
In this context, ε is the dielectric constant, A is the capacitor surface formed by the electrodes and d is the distance between the plates of the electrodes. Typical read-out circuits for recording these capacitive signal changes use switched capacitor circuit engineering in order to convert a charge generated by reference voltages, that is a function of the capacitive signal change, by integration to a voltage, and subsequently to a digital pulse code-modulated signal (PCM signal). In this context, there are generally two architectures for converting the analog capacitive signal to a voltage and subsequently to digitize it. Both approaches, in this context, generate a conversion of the capacitive signal to a voltage by a charge amplifier.
In the first approach, this voltage is digitized. In this connection, there is the possibility of undertaking a Nyquist conversion after filtering the voltage, or a modulation conversion of the voltage without filtering. In the case of the modulation conversion, the differential capacitor can be a component of the signal feed structure of a classical sigma delta converter.
In the second approach, the signal is digitized by a modulated, mechanically closed control circuit with the aid of a sigma delta modulation. In this context, the controlled variable is the resulting dynamic effect on the seismic mass of the differential capacitor.